There is an increasing demand for hard disk drives of higher density and information storage media as a prime product are also required to have a medium recording surface of high precision and high quality in order to achieve high recording density. A main component thereof is a glass substrate for hard disks, and accordingly, it is also essentially required to have high quality.
Glass substrates for hard disks are produced in a method including a polishing process to ensure high geometrical surface accuracy, and to effectively provide surface morphology in different frequency ranges, such as surface roughness, fine undulation and undulation, slurries and polishing pads having different working abilities have been effectively combined to provide a polishing process having two or more steps to achieve quality in geometry of significantly high accuracy.
The polishing process is generally divided into coarse polishing and precision polishing, and the coarse polishing is done with a polishing agent containing cerium oxide (60%) as a main component. In recent years, a polishing agent of cerium oxide of high purity has been developed, and it allows improved working rates and reduced flaws.
On the other hand, hard disk drives are required to have further higher density, and to achieve high recording density, a magnetic head has introduced therein a technique referred to as a dynamic flying height (DFH) mechanism. More specifically, on a head slider surface a recording and reproducing element is alone extended from a controlled flying height through thermal expansion toward a surface of a medium to achieve a further shorter effective distance and an increased magnetic signal intensity to thus allow a configuration of higher density to also achieve a significantly steady recording and reproduction. Furthermore, hard disk drives are increasingly mounted in mobile equipment, vehicles and the like and thus applied in more fields, and accordingly, increasingly required to be reliable in strength including non-operating shock resistance.
Japanese Patent Laying-Open No. 2010-030041 (PTL 1) and Japanese Patent Laying-Open No. 2008-088325 (PTL 2) disclose using a polishing agent of cerium of high purity. Using a polishing agent of cerium of high purity is expected to contribute to an increased polishing rate, reduced flaws, and the like.
Japanese Patent Laying-Open No. 2002-109727 (PTL 3) describes a method using cerium of high purity as a polishing agent to polish a substrate. The polishing agent disclosed does not contain fluorine.
When cerium oxide is used to provide coarse polishing, a hard and significantly efficient pad, such as a cerium oxide containing, foamed polyurethane pad, that can ensure a high working rate is used for the purpose of shaping as well as removing a surface layer damaged in a lapping step or a similar preworking step.
However, as hard and coarse particles of cerium oxide are contained, the polishing pad per se provides mechanical damage, and the coarse polishing step using cerium oxide per se cannot provide a completely homogenous and uniform surface.
On the other hand, conventionally, to provide a substrate enhanced in shock resistance, a slurry of fine cerium oxide, colloidal silica, zirconium or the like is used to perform a final polishing step and subsequently an ion exchange reaction of an alkaline element is utilized to perform a chemical strengthening process. However, as media have increased recording densities, it is now essential to reduce/prevent defects adhering to a surface of a substrate, and accordingly, it is now a mainstream that the chemical strengthening process is followed by a finishing step to polish approximately 2 μm or smaller.